1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the present invention relates to an exhaust gas purification device includes a NO.sub.x absorbent for removing NO.sub.x in the exhaust gas of an internal combustion engine.
2. Description of the Related Art
An exhaust gas purification device utilizing a NO.sub.x absorbent in order to remove NO.sub.x in the exhaust gas of an internal combustion engine is known in the art. The NO.sub.x absorbent, for example, contains precious metals such as platinum (Pt), and at least one substance selected, for example, from alkaline metals such as potassium (K), sodium (Na), lithium (Li), and cesium (Cs), alkaline earth metals such as barium (Ba) and calcium (Ca), and rare earth elements such as lanthanum (La) and yttrium (Y). The NO.sub.x absorbent absorbs NO.sub.x in the exhaust gas when the air-fuel ratio of the exhaust gas flowing through the NO.sub.x absorbent is lean, and releases the absorbed NO.sub.x when the oxygen concentration in the exhaust gas drops and, at the same time, reduces the absorbed NO.sub.x to N.sub.2. Further, in this specification, the term "air-fuel ratio of the exhaust gas" means a ratio of the air and the fuel, which are supplied to the engine or exhaust passages upstream of the NO.sub.x absorbent. Therefore, when no air and fuel is supplied in the exhaust passages upstream of the NO.sub.x absorbent 5, the air-fuel ratio of the exhaust gas becomes the same as the operating air-fuel ratio of the engine (i.e., the air-fuel ratio of the air-fuel mixture supplied to combustion chambers of the engine).
The mechanism of the absorbing and releasing operation of NO.sub.x by the NO.sub.x absorbent will be explained later in detail. However, the NO.sub.x absorbent also absorbs SO.sub.x by a mechanism the same as the absorbing mechanism of NO.sub.x when SO.sub.x is contained in the exhaust gas (in this specification, sulfur oxide such as SO2 and SO3 are referred to as SO.sub.x in general). However, since SO.sub.x absorbed in the NO.sub.x absorbent forms a stable sulfate, it is relatively difficult to make the NO.sub.x absorbent release the absorbed SO.sub.x. Therefore, the absorbed SO.sub.x tends to accumulate in the NO.sub.x absorbent. When the amount of SO.sub.x absorbed in the NO.sub.x absorbent increases, the capacity of NO.sub.x absorbent for absorbing NO.sub.x becomes low. This causes a deterioration of the ability of NO.sub.x absorbent for purifying NO.sub.x in the exhaust gas (hereinafter, this deterioration caused by the accumulation of SO.sub.x in the NO.sub.x absorbent is referred to as "a sulfur deterioration" or "an S-deterioration"). Particularly, in the case where a diesel engine is used, the S-deterioration becomes a problem since diesel fuel used for a diesel engine contains a relatively large amount of sulfur.
It is also known that SO.sub.x absorbed in the NO.sub.x absorbent can be released from the NO.sub.x absorbent by a mechanism the same as the mechanism for releasing NO.sub.x. However, it is difficult to release SO.sub.x from the NO.sub.x absorbent in the same conditions as the NO.sub.x releasing operation. Since SO.sub.x absorbed in the NO.sub.x absorbent forms a stable sulfate, it is required to raise the temperature of the NO.sub.x absorbent to a temperature higher (for example, about 500.degree. C.) than the temperature during the normal NO.sub.x releasing operation of the NO.sub.x absorbent (for example, about 250.degree. C.) under a reducing atmosphere (i.e., in a rich air-fuel ratio exhaust gas). This kind of SO.sub.x releasing operation for recovering the NO.sub.x absorbing capacity of the NO.sub.x absorbent from the S-deterioration is hereinafter referred to as "a SO.sub.x recovery operation" or "a S-recovery operation".
Japanese Unexamined Patent Publication (Kokai) No. 6-88518 discloses an exhaust gas purification device which performs the SO.sub.x recovery operation of a NO.sub.x absorbent. The device in the '518 publication includes a NO.sub.x absorbent disposed in the exhaust gas passage of an internal combustion engine for removing NO.sub.x from the exhaust gas. The device in the '518 publication performs the SO.sub.x recovery operation by controlling the air-fuel ratio of the engine to a rich air-fuel ratio intermittently (or continuously) when the engine is operated at a lean air-fuel ratio and at a high exhaust gas temperature.
However, since the device in the '518 publication starts the SO.sub.x recovery operation after the exhaust gas temperature becomes high when the air-fuel ratio of the exhaust gas is lean, a high temperature exhaust gas of a lean air-fuel ratio always flows into the NO.sub.x absorbent before the SO.sub.x recovery operation starts. As explained before, the NO.sub.x absorbent holds the absorbed SO.sub.x therein in a form of relatively stable sulfate particles. When the sulfate particles are exposed to a lean air-fuel ratio atmosphere at a high temperature, growth of the particles due to sintering occurs and the sizes of the sulfate particles become larger. When the sizes of the sulfate particles become large, it is difficult to decompose the sulfate particles by the SO.sub.x recovery operation since the stability of the sulfate particles largely increases. Therefore, if the NO.sub.x absorbent is exposed to a lean air-fuel ratio atmosphere at a high temperature always before the start of the SO.sub.x recovery operation as in the device of the '518 publication, the growth of the sulfate particles occurs and, thereby, S-deterioration of the NO.sub.x absorbent cannot be recovered by the SO.sub.x recovery operation due to the increased stability of the sulfate particles.
Further, if a particulate filter which traps particulate matter (for example, particulate in the exhaust gas of a diesel engine) is provided in the exhaust gas passage upstream of the NO.sub.x absorbent, it becomes necessary to burn the particulate matter trapped in the particulate filter periodically. Normally, the burning operation of the particulate matter is performed in a lean air-fuel ratio atmosphere. (Hereinafter, this burning operation of the particulate matter in the particulate filter is referred to as "a regenerating operation of the particulate filter".) Therefore, when the regenerating operation of the particulate filter upstream of the NO.sub.x absorbent is performed, high temperature exhaust gas of a lean air-fuel ratio continuously flows into the NO.sub.x absorbent and, thereby, the growth of the sulfate particles in the NO.sub.x absorbent occurs. This causes the problem of the difficulty in the SO.sub.x recovery operation of the NO.sub.x absorbent as explained above.
A similar problem occurs when a three-way catalyst or an oxidizing catalyst is provided in the exhaust gas passage upstream of the NO.sub.x absorbent in order to remove (oxidize) specific components in the exhaust gas such as HC and CO, or SOF (soluble organic fraction).
The exhaust gas of an internal combustion engine, especially, the exhaust gas of a diesel engine contains SOF (soluble organic fraction) composed of hydrocarbons such as unburned fuel, unburned lubricating oil, etc.
During the engine operation, a portion of the SOF in the exhaust gas is oxidized by the oxidizing catalyst or the three-way catalyst, and the rest of the SOF attaches to the surface of the catalyst. When the amount of SOF attached to the surface of the catalyst increases, the catalytic components are covered by SOF and the ability of the catalyst is deteriorated. This type of the deterioration of the oxidizing catalyst or the three-way catalyst is hereinafter referred to as "the SOF-deterioration of the catalyst". When the amount of SOF in the exhaust gas is large, therefore, it is necessary to burn the SOF attached to the surface of the catalyst periodically in order to prevent the SOF-deterioration. The burning operation of the SOF attached to the catalyst (hereinafter referred to as "a SOF recovery operation of the catalyst") is normally performed in a lean air-fuel ratio atmosphere. Therefore, when the SOF recovery operation of the catalyst is performed, a high temperature exhaust gas of a lean air-fuel ratio continuously flows into the downstream NO.sub.x absorbent. This also causes a problem in the SO.sub.x recovery operation of the NO.sub.x absorbent.